1. Technical Field
This disclosure relates to distance and velocity measurements using carrier signals. This disclosure also relates to estimating or tracking a position of an object.
2. Description of Related Art
Radio-frequency (RF) ranging technology may be used to provide a distance and relative position between objects having communication radios without the need to take mechanical measurements. Some RF ranging systems calculate the distance between two objects based in part on the time it takes a radio signal to propagate between those objects. In air, radio signals propagate at a constant rate, roughly equal to the speed of light. Digital data communications and RF systems may employ a variety of methods to encode digital data within an RF carrier signal.
Knowing the absolute position of a sufficient number of objects (such as, for example, cellular telephone towers fixed relative to the earth), an RF ranging system can be used to determine the absolute position of other radio-containing objects. Some RF ranging systems may utilize a signal from the Global Positioning System (GPS) to verify absolute location. In many situations, however, GPS signals are either unavailable or actively denied to a potential user. RF ranging systems can provide accurate information regarding distance or location even in locations or situations without access to GPS signals. Some positioning systems, for example, GPS-based systems, measure a time difference of arrival (TDOA) of several synchronized signals to aid in position calculations.
One problem with GPS-based tracking devices is that when GPS is unavailable, the system can no longer track the object with a high degree of accuracy. GPS may be unavailable in various locations: underground or other locations below surrounding terrain, such as mines, canyons, caves, tunnels, bunkers, and basements; urban locations, such as in or between skyscrapers and other large buildings; or locations with active interference of GPS signals or high levels of electromagnetic interference, such as signal jamming.
To compensate for the lack of GPS, some measurement systems rely on “dead reckoning” to estimate the position of an object. In dead reckoning, the current position of an object is estimated by measuring the course, speed, and time elapsed since the object was in a known prior position. For example, a person who is orienteering may utilize dead reckoning. By multiplying their average speed by the elapsed time, the person can estimate a total distance travelled. Using a map, the person can thus plot their path from a known starting position along a measured compass course. Dead reckoning, however, is only as reliable as the data used. In the orienteering example, error may be introduced in the estimates of the course travelled, the average speed, or the elapsed time. Some dead reckoning technology uses inertial navigation systems, which may include inertial measurement units, to track the position of an object over time. Although inertial navigation systems helped address some sources of error, they are still prone to inaccurate measurements and navigation errors.
Some dead reckoning systems include RF ranging systems that may utilize a round-trip time-of-flight measurement to compute the distance between two radios. These types of systems can be further classified into round-trip “full-duplex” configurations and round-trip “half-duplex” configurations. An example of half-duplex and full-duplex round-trip time-of-flight measurements is described in U.S. Pat. Nos. 8,199,047 and 8,314,731 (which are assigned to the same assignee as the present disclosure).
In a round-trip half-duplex configuration, a first radio transmits a signal to a second radio, which then performs calculations using that signal. The second radio then transmits a new signal, which may contain the results of the calculations performed by the second radio, back to the first radio. The first radio then utilizes the data from the second radio and other data within the first radio to calculate the round-trip signal propagation time. The system multiplies this time by the speed of light and divides by two to estimate the distance between the two radios.
Because round-trip range measurements between radios require each radio to transmit a signal to the other radio, errors may be introduced by differences between the transmitted baseband frequencies or carrier frequencies of each radio. For example, even radios with the same nominal frequency (e.g., 2.4 GHz) will have slight variations in their actual transmission frequencies due to the manufacturing tolerances or local oscillator drift or instability. Errors may also be introduced by the relative motion between the radios, resulting in Doppler shifts in the transmitted signals. These variations cause errors in measurements that may compound over time, leading to tracking or navigation errors.
When determining the position of an object, calculations may be performed by a data processor. The calculations may inherently produce finite errors in the real-time estimates of the position, velocity, and attitude, collectively known as “navigation errors.” If uncorrected, these errors grow unbounded with time. To help bound navigation errors, it is common in the art to employ various filtering techniques. One class of filters is known as Kalman filters. The term “Kalman filter” will be used to collectively refer to members and variants in this class of filters, including but not limited to extended Kalman filters (EKF) and unscented Kalman filters (UKF). Another class of filters includes particle filters.
Another source of errors in RF navigation systems is multipath interference. These types of errors are introduced when a signal transmitted between two radios reflects off of objects, such as the ground, buildings, walls, or vehicles. These signals are also received by the receiving radio, but followed a different path to reach the receiving radio. Multipath interference introduces errors or inaccurate measurements by artificially increasing the perceived distance between objects.
Although improvements to navigation systems and measurement systems have been developed over time, there remains a need for an improved tracking and location system that obviates or at least mitigates one or more of the shortcomings of previous techniques to allow more accurate computation of the current or real-time position.